(1) Field of the Invention
This invention relates to a corrosion resistant rare earth metal magnet, and more particularly relates to a rare earth metal-transition metal type magnet alloy having excellent coercive force and squareness and further having excellent corrosion resistance and temperature characteristics. The term "rare earth metal" used herein means Y and lanthanoid.
(2) Related Art Statement
Typical permanent magnets produced at the present time are alnico magnets, ferrite magnets, rare earth metal magnets and the like. The alnico magnet has been predominantly used for a long period of time in the magnet material field. However, the demand for the alnico magnet is recently decreasing due to the temporary rising of the price of cobalt, contained as one component in the alnico magnet, in the past because of its short supply and also due to the developments of inexpensive ferrite magnets and rare earth metal magnets having magnetic properties superior to those of alnico magnets. As for the ferrite magnet, it consists mainly of iron oxide and is consequently inexpensive and chemically stable. Therefore, the ferrite magnet is predominantly used at present, but it has a drawback that the ferrite magnet is small in maximum energy product.
There has been proposed an Sm-Co type magnet which is featured by both the magnetic anisotropy inherent to rare earth metal ions and the magnetic moment inherent to transition metals and has a maximum energy product remarkably larger than that of conventional magnets. However, the Sm-Co type magnet consists mainly of Sm and Co which are considered scarce natural resources, and therefore the Sm-Co type magnet is expensive.
In order to eliminate the drawbacks of the Sm-Co type magnet, it has been attempted to develop an inexpensive magnet alloy which does not contain expensive Sm and Co, but has excellent magnetic properties. Sagawa et al disclose ternary stable magnet alloys produced through a powder-sinter method in Japanese Patent Application Publication No. 61-34,242 and Japanese Patent Laid-open Application No. 59-132,104. J. J. Croat et al disclose a magnet alloy having high coercive force through a melt-spinning method in Japanese Patent Laid-open Application No. 59-64,739. These magnet alloys are Nd-Fe-B ternary alloys. Among them, the Nd-Fe-B magnet alloy produced through a powder-sinter method has a maximum energy product higher than that of the Sm-Co type magnet.
However, the Nd-Fe-B type magnet contains large amounts of reactive light rare earth metals, such as Nd and the like, and easily corrodible Fe as components. Therefore, the Nd-Fe-B type magnet is poor in corrosion resistance, and hence the magnet is deteriorated in its magnetic properties with the lapse of time, and is poor in reliability as an industrial material.
In general, in order to improve the corrosion resistance of the Nd-Fe-B type magnet, the sintered type magnet is subjected to a surface treatment, such as plating, coating or the like, while the resin-bonded type magnet is made from magnet powder subjected to surface treatment before its kneading together with resin powder. However, these anti-rust treatments cannot give an anti-rust effect durable for a long period of time to a magnet, and moreover the resulting magnet is expensive due to the necessity of the anti-rust treatment. Further, there is a loss of magnetic flux in the magnet due to the thick protective film. Therefore, conventional Nd-Fe-B type magnets have not hitherto been widely used due to these drawbacks.
In addition to such a drawback, the Nd-Fe-B type magnet is poor in temperature characteristics due to its low Curie temperature of about 300.degree. C. For example, the Nd-Fe-B type magnet has a reversible temperature coefficient of residual magnetic flux density of -0.12--0.19(%/.degree.C.), and is noticeably inferior to the Sm-Co type magnet having a Curie temperature of 700.degree. C. or higher and a reversible temperature coefficient of residual magnetic flux density of -0.03--0.04(%/.degree.C.). Accordingly, the Nd-Fe-B type magnet must be used at a lower temperature range compared to the Sm-Co type magnet and under an environment which does not oxidize and corrode the magnet, in order to satisfactorily utilize its excellent magnetic properties. That is, the use field of the Nd-Fe-B type magnet has hitherto been limited to a narrow range.
The present invention advantageously solves the above described problems and provides a rare earth metal-transition metal type magnet alloy having not only excellent magnetic properties but also excellent temperature characteristics and corrosion resistance.
The present invention is based on the results of the following studies.
There are two methods for improving the corrosion resistance of alloy. In one of the methods, a shaped body of the alloy is subjected to a surface treatment, such as plating, coating or the like, in order not to expose the shaped body to a corrosive and oxidizing atmosphere. In the other method, a metal element which acts to enhance the corrosion resistance of the resulting alloy is used. In the former method, additional treating steps for the surface treatment must be carried out in the production process, and hence the resulting alloy is expensive. Moreover, when the alloy surface is once broken, the alloy is corroded from the broken portion, and the alloy shaped body is fatally damaged due to the absence of countermeasures against the spread of the corrosion at present. While, in the latter method, the resulting alloy itself has a corrosion resistance, and hence it is not necessary to carry out the surface treatment of the resulting alloy. As the metal element which acts to enhance the corrosion resistance of an alloy by alloying, there can be used Cr, Ni and the like. When Cr is used, the resulting alloy is always poor in magnetic properties, particularly in residual magnetic flux density. While, the use of a ferromagnetic metal of Ni can be expected to improve the corrosion resistance of the resulting alloy without noticeably deteriorating its residual magnetic flux density.
The inventors have found out that, when at least 20% of Fe in an Nd-Fe-B magnet is replaced by Ni, the corrosion resistance of the magnet is remarkably improved, but the coercive force of the magnet is concurrently noticeably deteriorated. That is, even when the corrosion resistance of a magnet is improved, if the magnetic properties, which are the most important properties, of the magnet are deteriorated, the magnet can not be used for practical purposes.
The inventors have further made various investigations in order to improve the corrosion resistance and temperature characteristics of an Nd-Fe-B type magnet without deteriorating the magnetic properties demanded to the magnet as fundamental properties, and have found out that, when Ni is contained together with Co in an Nd-Fe-B magnet, that is, when a part of Fe in an Nd-Fe-B magnet is replaced by given amounts of Ni and Co, the above described object can be attained. The present invention is based on this discovery.